Consider an experiment in which participants take a pill that is either a medication or a placebo. The researchers want to determine the effect of the medication on depression. In this example, the control group would be

The control group in an experiment where there is a placebo present is the group taking the placebo. With depression you want to see noticeable changes in a person on new medication to see how they will react. Although the placebo group won't know it's a placebo there still could be some noted effects but for the most part the placebo group should remain mostly unaffected and therefore is the control.